1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus using the optical scanning device. In particular, the present invention relates to an optical scanning device suitably used for an image forming apparatus such as a digital copying machine or a laser beam printer employing an electrophotographic process, or a multi-function printer, in which light beam emitted from light source means is reflectively deflected by a polygon mirror as an optical deflector, a surface to be scanned is optically scanned with the light beam through a scanning lens system (imaging optical system), and image information is recorded.
2. Related Background Art
In recent years overfilled scanning optical systems (OFS scanning optical systems) have been utilized in optical scanning devices used in image forming apparatus such as laser beam printers, digital copying machines, and multi-function printers in order to deal with devices with higher speed and resolution, the system using a polygon mirror having a number of reflection surfaces (deflection surfaces) serving as deflection means, although their diameters are small.
With the overfilled scanning optical system, multiple surfaces can be achieved without making the polygon mirror size large, and it becomes possible to lighten a load of a motor that drives the polygon mirror and to perform high-speed scanning.
However, the overfilled scanning optical system has a problem in that the width of the reflected light beam from the polygon mirror in a main scanning direction changes together with angle of view (scanning angle), with the result that a spot size and a light amount distribution on the surface to be scanned become non-uniform across the scanning direction.
FIG. 7 is a diagram that shows a light beam state in the vicinity of an optical deflector in an optical scanning device using this type of overfilled scanning optical system.
An incident light beam 71 from a light source means (not shown) indicated by a dotted line in FIG. 7 is made incident at a larger width than the width in a main scanning direction of one facet (reflection surface) 76a of a polygon mirror 76 used as an optical deflector. A portion of the incident light beam 71 is then reflectively deflected by the facet 76a into a reflected light beam 72, and is guided to a surface to be scanned (not shown) through an imaging optical system. The width of the reflected light beam 72 in a main scanning direction at this point changes according to an angle φ at which the light beam is reflectively deflected by the polygon mirror 76. Even if an intensity distribution of the incident light beam 71 is taken as being constant along the width, the width of the reflected light beam 72 becomes narrower together with the rotation angle φ of the polygon mirror 76. As a result, an amount of the light reaching the surface to be scanned becomes smaller as the angle of view (scanning angle) becomes smaller, and the light amount distribution becomes non-uniform.
A variety of optical scanning devices in which the non-uniform light amount distribution is corrected have conventionally been proposed (refer, for example, to Japanese Patent Application Laid-Open Nos. 08-160338, 2001-108926 and 2001-125033).
In Japanese Patent Application Laid-Open No. 08-160338, a non-uniform light amount distribution on a surface to be scanned is corrected by inserting a filter having a transmittance distribution in a light beam that is entered to a polygon mirror. Further, in Japanese Patent Application Laid-Open Nos. 2001-108926 and 2001-125033, a non-uniform light amount distribution on a surface to be scanned is corrected by inserting a slit in a light beam that is entered to a polygon mirror.
In the publications described above, as for Japanese Patent Application Laid-Open Nos. 08-160338 and 2001-108926, correction is only performed with the objective of making the light amount distribution in the main scanning direction substantially constant on the surface to be scanned, out of consideration of changes in spot shape or changes in peak intensity due to changes in an F number of the light beam width in the main scanning direction. In Japanese Patent Application Laid-Open Nos. 2001-125033, two concepts of making the light amount distribution substantially constant by using a slit, and suppressing changes in the spot size surface area are disclosed. However, cases in which the two concepts are to be satisfied at the same time are not considered. Specifically, even if the amount of light of the reflected light beam 72 is made constant irrespective of the angle of view according to the method described above, the spot size varies on the surface to be scanned because the light beam width does not become constant. As a result, the peak intensity of the spot (imaging spot) varies despite the constant light amount. That is, for cases where an off-axis light beam width in a sub-scanning direction is expanded and its light beam amount is made substantially the same as an on-axis light beam amount, the spot in the sub-scanning direction becomes smaller than necessary, with the result that the peak intensity of the spot becomes larger than that on-axis.
In this state an exposure amount becomes constant for cases where the light beam is continuously kept on in an effective scanning region, but differences in printing quality develop according to the angle of view for cases where minute points are printed in order to form minute characters or halftone images. That is, the maximum value of the printing dot exposure distribution differs from that on-axis when the angle of view becomes large, and problems arise, for example, solid printing occurs or printing itself cannot be performed.